The invention concerns a process for separation of the constituents of a mixture in the gas phase using a composite membrane comprising an adsorbent and, in particular, to the separation of a mixture comprising hydrocarbons.
Separation processes using membranes and gas permeation processes in particular have recently undergone great advances due to the development of asymmetric membranes whose active layer is very thin and of hollow fiber modules with large specific surfaces.
Nonetheless, these processes, which already have commercial applications, for example, in the separation of hydrogen from a gaseous mixture of hydrogen and hydrocarbons or in the separation of carbon dioxide from a gaseous mixture of carbon dioxide and hydrocarbons, have not yet led to results in the field of separation of hydrocarbon isomers.
In fact, the polymer-based membranes which have been developed so far do not show sufficient selectivity for the separation of isomers such as normal paraffins/isoparaffins, meta and paraxylene, olefins/paraffins.
The composite membranes according to previous techniques involve a superposition of parallel layers formed from materials with differing properties, according to the diagram in FIG. 1 in which these parallel layers (1, 2, 3) are represented.
For example, U.S. Pat. No. 4,230,463 describes how a very permeable and not very selective silicone membrane can be superimposed on an active polysulfone layer in order to improve performance, thus allowing reduction of the microfissure effect in the active polysulfone layer.
According to this principle, use of a composite membrane comprising an active layer consisting of a material such as a zeolite has already been suggested in European patent EP-0810200.
In practice, it is nonetheless difficult to make up a crystalline layer from a material such as a zeolite that is sufficiently fine and uniform.
European patent No. 0180200 points out that in order to make up such a selective layer by impregnation, trapping zeolite particles in the pores of the porous medium, the pores must be sufficiently well-calibrated and the particles must have a relatively uniform dimension. This is also the case in U.S. Pat. No. 3,567,666 which describes a porous, polymer-based structure whose channels contain molecular sieve microporous particles, the pores of these particles being interconnected by the channels.
These methods encounter numerous difficulties: difficulty in calibrating the pores of the porous support and the zeolite particles, in controlling the thickness of the zeolite film and in preparing a continuous layer.
Moreover, it can be difficult to carry out various treatments after trapping particles, particularly if the porous support is polymer-based and if the treatment necessitates high temperatures.
FR patent No. 2,079,460 describes a membrane consisting of (a) a polymer, (b) a zeolite dispersed in the polymer and (c) a strengthening support such as a polyester or a polyamide. The thickness of membranes obtained in this way is between 100 and 1000 micrometres. Due to this, diffusion of the product that is passed across the membrane is low and the rate of flow of products are reduced. We are thus led to increase the difference in pressure between faces of the membrane in order to obtain an acceptable outlet flowrate. However, this leads to an increased cost in materials as well as an equally costly consumption of energy and an increased risk of membrane rupture.
A membrane comprising a polymer matrix consisting of a silicone (silicone rubber) or polyisoprene-based elastomer, in which a zeolitic material is dispersed, is described in the patent EP-A-0.254.758 and in the document "Journal of Membrane Science vol. 35, n. 1, Dec. 15, 1987, pages 39-55, Elsevier Science Publishers B. V. Amsterdam".
Further, a membrane comprising functional groups or zeolites chemically grafted onto polymer molecules is described in the patent EP-A-0.154.248.
Moreover, previous techniques illustrated by U.S. Pat. No. 2,924,630 describe a separation process using membranes containing zeolites whose pore diameters are between 3 and 15 Angstroms. Such techniques are also described in the document Chem. Abstract, Volume 85, 1976 n. 1, page 410, n. 5102Z, which describes a molecular sieve membrane used for separation of methane and lower olefins.
Finally, previous techniques are illustrated by the document Patent Abstract of Japan, volume 9, n. 317 (2040), 12 Dec. 1985 which describes a membrane with silica whose surface is treated with a binding agent and by FR patent No. 2011125 where the membrane only comprises one layer of granules imbedded partially in a plastic sheet.
It has been discovered that separation of hydrocarbon isomers becomes possible if a new membrane is formed.
Furthermore, the process according to the invention allows separation of gaseous mixtures, such as hydrogen/methane and methane/CO.sub.2, to be carried out under more favorable conditions.
The membrane used according to the invention is a composite membrane incorporating a selective adsorbent phase vis-a-vis one of the constituents present in the mixture to be separated.